A179956 - OEIS

%I #12 Aug 05 2019 02:57:22

%S 1,21,121,1121,11121,111121,1111121,11111121,111111121,1111111121,

%T 11111111121,111111111121,1111111111121,11111111111121,

%U 111111111111121,1111111111111121,11111111111111121,111111111111111121,1111111111111111121,11111111111111111121

%N a(n) is the smallest integer that contains a(n-1) but does not begin with a(n-1).

%F For n > 2, a(n) = 10^(n-1) + a(n-1). - _Jon Maiga_, Aug 04 2019

%e The next term after 1 is 21, because 21 is the lowest integer higher than 1 that contains a 1 but does not start with 1.

%t a[1] = 1

%t a[2] = 21

%t a[n_] := 10^(n - 1) + a[n - 1]

%t Array[a, 20] (* _Jon Maiga_, Aug 04 2019 *)

%K easy,base,nonn

%O 1,2

%A _Dominick Cancilla_, Aug 03 2010

%E Definition corrected and more terms added by _Jon Maiga_, Aug 04 2019